THE FOUR EXrEKIMENTAL METHODS. 231 



distance everything goes on precisely as it would do if the mountain 

 exercised no influence whatever, which^ accordingly, we, with sullicient 

 reason, conclude to be the tiict. ' 



The difliculty, therefore, in applying the methods already treated of 

 to determine the eflects of Permanent Causes, is confined to the cases 

 in which it is impossible for us to get out of the local limits of their 

 influence. The pendulum can be removed from the influence of the 

 mountain, but it cannot be removed fi-om the influence of the eart^h : 

 •we cannot take away the earth from the ^)endulum, nor the pendulum 

 from the earth, to ascertain whether it would continue to vibrate if the 

 action which the earth exerts upon it were withdrawn. On what 

 evidfeuce, then, do we ascribe its vibrations to the earth's influence 1 

 Not on any sanctioned by the Method of Difference ; for t)ne of 

 the tAVo instances, the negative instance, is wanting. Nor by the 

 Method of Agreement ; for although all j)endulums agree in this, that 

 during their oscillations the earth is always present, why may we not 

 as well ascribe the phenomenon to the sun, which is equally a co- 

 existent fact in all the experiments 1 It is evident that to establish 

 even so simple a fact of causation as this, there was required some 

 method over and above those which we have yet examined. 



As another example, let us take the phenomenon Heat. Independ- 

 ently of all hypothesis as to the real nature of the agency so called, 

 this fact is certain, that we are unable to exhaust any body of the whole 

 of its heat. It is equally certain that no one ever perceived heat not 

 emanating from a body. Being unable, then, to separate Body and Heat, 

 we cannot eflfect such a variation of circumstances as the foregoing three 

 methods require; we cannot ascertain, by those methods, what por- 

 tions of the phenomena exhibited by any body are due to the heat con- 

 tained in it. If we could observe a bordy with its heat, and the same 

 body entirely divested of heat, the Method of Difference would show 

 the effect due to the heat, apart from that due to the body. If we 

 could observe heat under circumstances agreeing in nothing but heat, 

 and therefore not characterized also by the presence of a body, we 

 could ascertain the effects of heat, from an instance of heat with a body 

 and an instance of heat without a body, by the Method of Agi-eement ; 

 or, if we pleased, we could determine by the Method of Difference 

 what effect was due to the body, when the remainder which was due to 

 the heat would be given by the Method of Residues. But we can do 

 none of these things ; and without them the application of any of the 

 three methods to the solution of this problem would be illusory. It 

 woidd be idle, for instance, to attempt to ascertain the effect of heat by 

 subtracting from the phenomena exhibited by a body, afl that is due to 

 its other properties ; for as we have never been able to observe any 

 bodies without a portion of heat in them, the effects due to that heat 

 may fonn a part of the very results, which we affect to subtract in order 

 that the effect of heat may be sliowii by the residue. 



If. therefore, there were no other methods of experimental investi- 

 gation than these three, we should be for ever unable to determine 

 the effects due to heat as a cause. But we have still a resource. 

 Though we cannot exclude an antecedent altogether, we may be able 

 to produce, or nature may produce for us, some modification in it. By 

 a modification is here meant, a change in it, not amounting to its total 

 removal. If some modification in the antecedent A is always followed 



